Linear encoders for precise distance measurement are required in many fields of application in which the position of an element movable along a linear path, e.g. the position of a machine component on a linear axis, is to be determined. The positions detected in this case can be used as position values for measurement purposes, or else for positioning components by means of a drive with position control loop. Such linear position encoders are correspondingly found in apparatuses such as coordinate measuring machines (CMM), geodetic apparatuses, robot arms or hydraulic actuators.
For this purpose, a linear encoder comprises a scale and a read head for scanning or reading the scale, which are movable in a feed direction relative to one another, and also a control and evaluation unit for regulating measurement processes and for assigning a scanning signal generated by the read head to a position. In this case, dependent on requirements and structural possibilities, either the read head extending in the feed direction is stationary and the scale is movable, e.g. by the scale being connected to a movable object whose linear movement is intended to be detected; or the scale extending in the feed direction is fixedly positioned and the read head is moved relative thereto, for example by virtue of the fact that a measurement slide of a coordinate measuring machine that is provided with a read head is moved over a scale carrier fitted to a measurement table. In principle, different physical principles of action are suitable for scanning purposes, such as e.g. optical or capacitive scanning. In this case, capacitive linear encoders, compared with comparable optical linear encoders, afford the advantage of a lower power consumption and more cost-effective construction.
Incremental and absolute linear encoders are known. In the case of absolute systems, a position is directly assignable to each relative position of read head with respect to scale by virtue of the scale having over the entire measurement section an absolute position code composed of unique code words, which is assignable to exactly one position by a control and evaluation unit. In the case of linear encoders with incremental determination of positions, by contrast, the scanning signals are not unique, but rather are repeated many times over the entire measurement region. The distance to which an increment corresponds is stored in a control and evaluation unit of the linear encoder. Consequently, the distance covered during a relative movement of scale and read head and hence a relative position can be determined by counting down the increments. In order to locate such a relative position in absolute terms, a defined zero position as absolute reference point is taken as a basis during a relative movement. Such a zero position or zero point is defined by a position reference marker detectable by the read head on the scale (or on the read head in the case of stationary scale). In capacitive measurement systems, such a position reference marker is usually based on a magnetic or inductive principle of action, rather than on a capacitive principle of action.
EP 1173730 B1, by contrast, discloses a capacitive linear encoder comprising a position reference marker defining a zero point that is based on a capacitive principle of action. The linear encoder comprises a scale with rectangular receiver electrodes which are capacitively scannable by a read head by means of a rectangular receiver plate. As position reference marker that indexes the absolute reference point, use is made of two additional rectangular receiver electrodes adjacent to one another on the scale, which have a greater width, understood as extent in the feed direction, than the other receiver electrodes serving for defining the increments. The zero point is indexed by means of a joint evaluation both of the difference and of the sum of the capacitive signals which arise when scanning the two additional receiver electrodes.
However, particularly in the case of capacitive linear encoders having a relatively long measurement section, the absolute location of the positions on the basis of the increments counted down, proceeding from the zero point, is naturally susceptible to errors, for example as a result of erroneous increment quantities or errors when counting down the increments, e.g. as a result of noisy or disturbed scanning signals. In the case of linear encoders according to the prior art, avoiding or compensating for such erroneous absolute position values requires scales which are manufactured and mounted highly precisely, complex means for electrical shielding, as proposed e.g. in EP 1173730 B1, and/or complex error correction measures. Such measures are costly.